1. Field of the Invention
The present invention relates to a nickel-base superalloy, article made thereof and method for making such article. More particularly, the invention relates to such an article having a single crystal structure with an improved combination of mechanical properties and resistance to oxidation and hot corrosion.
2. Description of the Prior Art
The efficiency of gas turbine engines depends significantly on the operating temperature of the various engine components. Increased operating temperatures will result in higher efficiency engines. This requirement has led to the development of heat-resistant nickel-base superalloys which will withstand high temperatures and maintain their basic material properties at high temperatures. The requirement for increased operating temperatures has also led to the development of highly complex cast hollow shapes which will provide efficient cooling of the material used to produce such shapes.
The casting processes used with earlier nickel-base superalloys generally produced parts which contained equiaxed grains (nonoriented structures) of the type normally found in most cast metal articles. These earlier nickel-base superalloys are generally referred to as conventional nickel-base superalloys. Improvements in alloy performance, i.e., the ability to withstand higher temperature without impairing other needed qualities, were achieved through alloy development and the introduction of improved processing techniques. The strength of such alloys, and other important characteristics, were dependent upon the strengths of the grain boundaries developed during the casting process. The application of conventionally cast equiaxed articles was limited because the grain boundaries of the nickel-base alloys produced by the earlier casting methods could not be strengthened to the same level as the internal structure of the grains and eventually became weak links in the structure, limiting the potential strength and life of the alloy and article produced therefrom. To enhance such alloys, initial efforts were aimed at strengthening the grain boundaries by the addition of various grain boundary strengthening elements such as carbon (C), boron (B), zirconium (Zr), and hafnium (Hf).
Efforts to further develop strength levels in nickel-base superalloys by reducing or eliminating grain boundaries were initiated. Preferentially orienting the grains in the direction of the principal stress axis, which generally coincides with the longitudinal direction, to provide a columnar grain structure (grains oriented in a single direction, each having long, slender proportions) eliminates grain boundaries transverse to the growth direction. Such a structure provides increased strength and ductility along the principal stress axis. Such an oriented grain structure was found to be achievable by a process referred to as directional solidification which had long been used for other purposes, such as the development of magnets. That process has been described and improved upon, for instance, in U.S. Pat. No. 3,897,815-Smashey, issued Aug. 5, 1975, the disclosure of which is incorporated herein by reference.
When compared with conventionally cast alloy articles, the application of the directional solidification process has produced cast articles exhibiting increased strength in the longitudinal direction due to the elimination of grain boundaries. In addition, it has provided an increase in other properties, such as improved ductility and resistance to low cycle fatigue, due to such preferred grain orientation. However, reduced strength and ductility properties has still existed in the transverse directions due to the presence of columnar grain boundaries in such directionally solidified columnar structure articles. Additions of Hf, C, B, and Zr were utilized to improve the transverse grain boundary strength of such alloys as was done previously in conventional equiaxed nickel-base superalloys. However, these elemental additions also acted as melting point depressants. This resulted in limitations in alloying and in heat treatment which would not allow the development of maximum strengths within such directionally solidified alloys. The addition of Hf, C, B, and Zr was further found to compromise additional properties.
In U.S. Pat. No. 4,169,742-Wukusick, et al., issued Oct. 2, 1979, vanadium was found to be necessary in order to attain in the alloy article high stress rupture strengths with adequate castability (freedom from grain boundary cracking). However, vanadium can be detrimental to oxidation and corrosion resistance; therefore, a compromise in the balance of properties was required.
It has been recognized for some time that articles could be cast in various shapes as a single crystal, thus eliminating grain boundaries. Known single crystal technology was applied to eliminate the problems associated with grain boundaries in the transverse direction of columnar structured alloys by developing such single crystal articles (without grain boundaries). This elimination of grain boundaries provides greater freedom in alloying and an increase in alloy melting points. Such increases in the melting point of an alloy permits higher heat treatment temperatures, thus resulting in more effective strengthening of the alloy.
Early application of single crystals to turbine parts was performed using conventional nickel-base superalloys or modifications thereto. In order to achieve a satisfactory combination of properties and castability, use of such alloys as cast single crystal articles resulted in some sacrifice in the surface-related properties of oxidation and hot corrosion resistance. Conversely, other studies with that type of alloy have shown that significant improvements in oxidation and hot corrosion resistance were achievable only through reductions in strength and/or castability. Thus, there has been a need for an alloy specifically designed for casting as a single crystal and which produced an article having an improved combination of mechanical properties and improved resistance to oxidation and hot corrosion.